MUMBAI, India --Indian police are looking into "every possible hostile group" in their search for the culprits behind the triple bombing in the heart of Mumbai that killed 17 people and wounded 131 others, a senior minister said Thursday. The attacks were the worst terror strike in the country since the siege of Mumbai that killed 166 people 31 months ago, and government officials struggled to reassure Indians over their safety. "I want to assure everyone both in India and outside, that India will continue to work and grow and prosper," Home Minister Palaniappan Chidambaram said a news conference Thursday after an emergency security meeting. No one has claimed responsibility for Wednesday's attacks, which came just months after peace talks resumed between India and Pakistan. Indian officials have so far refused to speculate on who might be behind the blasts. "We are not pointing a finger at this stage," Chidambaram said. "We have to look at every possible hostile group and find out whether they are behind the blast." Chidambaram said Indian intelligence received no warning of a possible attack on Mumbai before the blasts. "Whoever has perpetrated this attack has worked in a very, very clandestine manner," he said. The bombs used in the separate attacks were made of ammonium nitrate and were not remotely triggered, he said. He lowered the casualty toll to 17 confirmed deaths and 131 injuries. He said a severed head was found that could be an 18th casualty. He did not explain the discrepancy from an earlier government statement that confirmed 21 deaths.